A. W. Ewell — Air in an Intense Electric Field. 375 



special conditions of high temperature (the arc discharge*), 

 and low pressure.f 



The following empirical formula agrees approximately with 

 the upper portion of the corrected curves (i. e., where the 

 current density exceeds 15 microamperes per square centi- 

 meter) : 



* = ™-V W (4) 



where i is the current per square centimeter in microamperes 

 and V is the electromotive force applied to the thickness, d, 

 of air. 



The effective values of the potentials at which appreciable 

 ionization commences and the maximum electric intensity at 

 the different distances are as follows : 



Air thickness ... -20 '43 i*31 2 '41 -30 1*66 



Ionization potential ._. 6000 9000 19000 30000 6000 22000 

 Ionization intensity (max.) ... 423000 29600 21200 17000 28200 18400 



The latter is given approximately by the formula : 



1 

 %/d 



E max =22,500 y7 = (5) 



The decrease in requisite intensity for ionization with 

 increase in distance is probably due to partial ionization.^: 



In fig. 5 are plotted the power factor and power absorbed 



for the different distances. [From fig. 2 it is readily seen that 



the power factor is the sine of the angle between A, the 



applied e.m.f., and B, which is a quarter period in advance 



E C 

 of the total current : = . _ ; and the power absorbed is the 



A B ' r 



product of the current, the potential applied to the electrodes, 



and the power factor.] 



The current heats the glass but little and therefore, before 



appreciable ionization begins the current and e.m.f. differ in 



phase by nearly a quarter period and the power factor is low. 



When ionization commences this part of the current in the air 



is in phase with the e.m.f. and hence the power factor 



increases. As the ionization current increases, however, the 



fraction of the total e.m.f. which is applied to the glass must 



greatly increase, and since in the glass, e.m.f. and current 



differ in phase by a quarter period, the power factor reaches a 



maximum and then decreases with increasing current. 



* Thomson, Conduction of Elec. through Gases, §214. 

 f Winkelmann, IV, 2ded., p. 519; Tovvnshend, Phil. Mag., March, 1905, 

 p. 289; Toepler. Ann. der Phys., xiv, p. 757, 1905. 



% Thomson, Conduction of Elec. through Gases, §197. 



